A ferromagnetic material that has little or no retentivity is said to be magnetically soft. When a magnetically soft material is magnetized in a magnetic field and then removed from that field, it loses most of the magnetism exhibited while in the field. A magnetically soft material is usually characterized by low hysteresis loss, a high magnetic permeability, and a high magnetic saturation induction. Magnetically soft materials are used in various static and rotating electrical devices, such as motors, generators, alternators, transformers, and magnetic bearings because of their desirable magnetic characteristics for such uses.
An iron-cobalt-vanadium alloy sold by Carpenter Technology Corporation under the trademark HIPERCO.RTM. Alloy 50 provides a very high saturation magnetization per unit weight of material. The nominal weight percent composition of the HIPERCO.RTM. Alloy 50 alloy is as follows.
______________________________________ wt. % ______________________________________ Carbon 0.01 Manganese 0.05 Silicon 0.05 Cobalt 48.75 Vanadium 1.90 Niobium 0.05 Iron Balance ______________________________________
HIPERCO.RTM. Alloy 50 alloy has been used in rotor and stator laminations in electrical generators because its use results in a significant reduction in the weight of such devices. The magnetic and mechanical properties of the alloy are highly dependent on the grain size of the alloy, which, in turn, is dependent on the alloy's composition and how it is annealed, particularly the annealing temperature. The small amount of niobium present in the HIPERCO.RTM. Alloy 50 alloy aids grain refinement which benefits the strength of the alloy. When the alloy is annealed at the lowest practical temperature (i.e., about 720.degree. C. (1328.degree. F.)), it provides a yield strength of up to 448 MPa (65 ksi) together with adequate magnetic properties. While a yield strength of 448 MPa (65 ksi) has heretofore been adequate, electrical generators and magnetic bearings are being designed for operating speeds in excess of 50,000 rpm. At such speeds a yield strength significantly higher than 448 MPa (65 ksi) is required.
U.S. Pat. No. 4,933,026 (Rawlings et al.) relates to a soft magnetic alloy having the following composition in weight percent.
______________________________________ wt. % ______________________________________ Carbon 0.03 max. Manganese 0.3 max. Silicon 0.1 max. Nickel 0.3 max. Cobalt 33-55 Vanadium No positive addition Tantalum + Niobium 0.15-0.5 Iron + Impurities Balance ______________________________________
The alloy described in the Rawlings et al. patent contains Ta and/or Nb in place of V for the alleged purpose of obtaining increased magnetic saturation induction. However, experience with the Rawlings et al. alloy has shown that the alloy has a relatively low electrical resistivity. Such low electrical resistivity results in undesirably high energy losses from eddy currents, as when the alloy is used in the rotor of a high speed generator which operates at very high flux reversal rates, e.g., about 5,000 Hz.
U.S. Pat. No. 3,634,072 (Ackermann et al.) relates to a magnetic alloy having the following composition in weight percent.
______________________________________ wt. % ______________________________________ Carbon 0.03 max. Manganese 0.8 max. Silicon 0.4 max. Phosphorus 0.02 max. Sulphur 0.02 max. Chromium 0.1 max. Nickel 0.8 max. Molybdenum 0.2 max. Cobalt 45-52 Vanadium 0.5-2.5 Niobium 0.02-0.5 Zirconium 0.07-0.3 Iron 45-52 ______________________________________
The alloy described in the Ackermann et al. patent contains one or both of 0.02-0.5% niobium and 0.07-0.3% zirconium to improve ductility without adversely affecting the magnetic properties of the alloy. An important characteristic of that alloy is that it can withstand long periods in the grain-growth temperature range without undergoing a significant loss in ductility. The grain-growth temperature range extends from just above the order-disorder temperature to about the ferrite-austenite transformation temperature. Finished forms of the material described in Ackermann et al. are given a final annealing heat treatment in dry hydrogen at a temperature ranging from 760.degree. to 843.degree. C. (1400.degree. to 1550.degree. F.) for 4 hours. However, an article produced in accordance with Ackermann et al. does not have a yield strength sufficiently high to be useful in high rotating speed electrical devices such as the aforementioned aerospace generators and magnetic bearings.
In view of the foregoing, there is a need for an annealed article that has both high yield strength and good electrical and magnetic properties to meet the demands imposed by the significantly higher speeds of the newest generation of electrical generators and magnetic bearings.